PREPARATION OF CHROMIUM-CONTAINING SUPPORTED CATALYSTS

Abstract: Chromium-containing supported catalysts having low carbon content and low Cr VI content are obtained by subjecting organic solvent containing inorganic oxide gel to a partial oxidation. The chromium-containing gel to be treated is preferably obtained by azeotropic distillation of a hydrogel. The resulting chromium-containing gel catalysts may be activated by calcination prior to use. The method is especially useful for production of chromium-containing polyolefin catalysts.

Description: PREPARATION OF CHROMIUM-CONTAINING SUPPORTED CATALYSTS Background of the Invention The present invention relates to the production of oxide gel-based supported catalysts, and more specifically to an improved method for preparing oxide 5 gel-based supported catalysts derived from aqueous hydrogels which have been treated with organic solvents. Often in commercial practice, oxide gel-based supported catalysts are made by forming an initial porous hydrogel in an aqueous medium. In order to render the 10 gel suitable for use in catalytic processes, it is necessary to remove water from the gel. Organic solvents are often used to treat the gel as part of the water removal process. The organic solvent must then be removed from the gel at a later stage in forming the 15 supported catalyst. U.S. 4,436,883 discloses a method for preparing silica-titanium-chromium polymerization catalysts wherein a silica-titanium-chromium hydrogel (tergel) is treated with a C 5 -C 6 alcohol to obtain a xerogel, and the xerogel 20 . is heated (calcined) in an inert atmosphere (N 2 ) to remove sorbed alcohol prior to activation in the presence of oxygen (activation) . While the above-noted process produces an active i olefin polymerization catalyst, it is found that the use 25 of a high temperature alcohol removal step in the presence of nitrogen frequently results in a catalyst that is unacceptably high in carbon. Furthermore, attempts to remove excess carbon from Cr III (trivalent)-containing catalysts during the manufacturing process by oxidation (heating in air) , may result in a catalyst that contains excessive quantities of toxic Cr VI (hexavalent) which is undesirable from the standpoint of material handling and shipping. Summary of the Invention It is therefore an object of the present invention to provide an improved process for manufacturing oxide gel-based supported chrominum-containing polymerization catalysts from organic solvent-treated hydrogels which results in reduced carbon content while avoiding the conversion of Cr III to Cr VI. It is a further object to provide a method by which the carbon content of chromium Ill-containing catalysts may be reduced to a level of below about one weight percent while substantially avoiding conversion of Cr III to Cr VI. It is still a further object to provide a method for preparing low carbon non-activated Cr III catalysts that may be readily packaged, shipped, and handled for use in an olefin polymerization process. These and still further objects will become readily apparent to one skilled-in-the-art from the following detailed description and specific examples. Detailed Description of the Invention Broadly, the invention contemplates supported catalyst manufacturing processes wherein an inorganic oxide xerogel, which has been obtained by treating a hydrogel with an organic solvent is subjected to controlled high temperature partial oxidation to remove sorbed organic solvent. More specifically, we have found that low carbon, non-activated inorganic oxide gel chrominum-containing catalysts may be produced by a process comprising: (1) Obtaining a chrominum-containing xerogel that contains sorbed organic solvent by a method involving treating an inorganic oxide hydrogel prepared in an aqueous medium with an organic solvent (typically 1- hexanol) ; (2) Subjecting the organic solvent-containing xerogel to partial oxidation at a temperature of about 450 to 700°C (preferably 510 to 677°C). The partial oxidation is conducted in an atmosphere that contains from about 15 to 30, preferably 18 to 26, volume percent air or about 3 to 6, preferably about 3.6 to 5.2, volume percent 0 2 mixed with an inert gas (typically nitrogen) . The partial oxidation is preferably conducted for a period of 5 to 15 minutes and preferably 8 to 12 minutes. About 0.1 to 0.5, preferably 0.2 to 0.4, parts by weight air or about 0.02 to 0.12, preferably 0.05 to 0.1, parts by weight 0 2 is mixed with each part by weight organic solvent contained in the xerogel. The oxide gel-based catalyst obtained by the above process contains less than one weight percent carbon, preferably below about 0.8 weight percent carbon. Chromium III is added to the gel by any known technique before the partial oxidation (for example, as a cogel component or by impregnation of the aqueous hydrogel) . The resulting unactivated xerogel catalyst from the process preferably has about 1.0% chromium VI based on the total chromium in the catalyst. The chromium-containing catalyst made by any of the above processes is preferably activated by heating in the presence of air at a temperature of about 400 to 900°C to convert the chromium III component to chromium VI. The oxide gels used may be any known inorganic oxide gel or multicomponent gel (e.g. cogel, tergel, etc.). Preferably, the gel is selected on the basis of its suitability as a catalyst or catalyst support. Preferred oxide gels are selected from the group consisting of silica, silica-titania, silica alumina, chromium-silica, silica-titania-chromium, aluminum phosphate, alumina and mixtures thereof. In the silica-titania-chromium system, the non- activated catalysts preferably contain the following range of components: Component Weight percent Si0 2 95 to 100 Ti 0 to 5 Cr 0 to 2 and the following physical properties: Pore Volume (N 2 ) 2.2 to 2.5 cc/g Surface Area 420 to 550 m 2 /g Density 0.18 to 0.25 g/cc The invention is also applicable to other chromium- containing systems as well as to silica-titania-chromium catalysts having properties different from those stated above. The inorganic oxide hydrogel used in the method of the invention may be formed by any known process. See for example U.S. Patents 3,652,214, 3,900,457, 4,152,503, and 5,292,701. Where a polyolefin catalyst is the intended use, preferably the hydrogel is prepared by a method known to yield a suitable gel for that purpose. Preferably, the step (1) treatment involves an azeotropic distillation using an organic solvent such as hexanol. The invention is also applicable where other organic solvent treatments, such as continuous extraction have been used to treat the hydrogel. The invention is not limited to any specific organic solvent. For azeotropic distillation, preferably the aqueous hydrogel is combined with an organic solvent (preferably 1- hexanol) in a reaction vessel in amounts ranging from 1.8 to 2.2 parts by weight hydrogel per part by weight solvent. The mixture is then heated to a temperature of 97 to 99°C to azeotropically distill the water in the hydrogel and to obtain a xerogel which contains about 25 to 30 weight percent sorbed organic solvent expressed as percent total volatiles. The xerogel is then preferably dried, ground and sized to a desired particle size range of about 40 to 300 microns. At step (2) , the sized xerogel is heated at a temperature of 510 to 677°C, typically in a controlled atmosphere rotary calciner, in the presence of an atmosphere which contains from about 18 to 26 percent air by volume in admixture with an inert gas such as nitrogen for a period of 5 to 15 minutes. During the heating, the sorbed solvent is oxidized to Cθ 2 and water thereby lowering the carbon content of the catalyst to below about one weight percent while avoiding any significant oxidation of Cr III to Cr VI. The resulting catalyst may be activated by heating in air and used in conventional olefin polymerization processes. Having described the basic concepts of our invention, the following examples are set forth to illustrate specific embodiments. EXAMPLE 1 2900 lbs of silica-titania-chromium hydrogel (12% Si0 2 ) was mixed with 1370 lbs of 1-hexanol. The mixture was then heated at 98°C to remove and azeotropically distill the water from the pores of the hydrogel. The resulting xerogel contained 25 percent 1-hexanol expressed as percent total volatiles. The hexanol-containing xerogel was heated in a rotating calciner maintained at a temperature of 510 to 677°C. The feed rate of the xerogel through the calciner was maintained at 125 lbs/hour while a mixture containing 22 volume percent air (4 SCFM) mixed with nitrogen (14 SCFM) at 18 SCFM total gas flow. The residence time of the xerogel in the calciner was 5 to 15 minutes. A comparison (prior art) sample was prepared using the same conditions except the inclusion of oxygen was omitted. The calcined xerogel samples were analyzed and found to possess the following chemical/physical properties: Chemical wt. %) A (Invention) B (Prior Art) Si0 2 96.3 96.3 Ti 2.52 2.48 Cr 0.97 0.98 C 0.80 1.5 Cr III 0.97 0.98 Cr VI <0.01 <0.01 Total volatiles 5.3 5.1 Na 2 0 0.04 0.06 Physical Total pore volume (cc/g N 2 ) 2.34 2.30 Density (g/cc) 0.21 0.21 Surface Area (m 2 /g N 2 ) 521 511

Claims: WHAT IS CLAIMED IS: 1. In a method for preparing inorganic oxide gel- based chromium-containing catalysts wherein an inorganic oxide hydrogel is treated with an organic solvent to obtain a xerogel which is calcined to remove said organic solvent, the improvement comprising: conducting said calcination at a temperature of 450 to 700°C in a gaseous atmosphere which comprises about 3 to 6 volume percent oxygen admixed with an inert gas for a period of about 5 to 15 minutes, whereby the carbon content of said xerogel is reduced to below about one weight percent. 2. The method of claim 1 wherein from about 0.02 to 0.12 parts by weight oxygen is included in said atmosphere per part by weight carbon contained in said xerogel. 3. The method of claim 3 wherein the xerogel contains up to about 30 weight percent organic solvent prior to said calcination. 4. The method of claim 1 wherein the xerogel to be calcined contains Cr III and the calcined product contains about one percent or less Cr VI based on the total Cr present in said xerogel. 5. The method of claim 1 wherein the inorganic oxide gel includes a gel selected from the group consisting of silica, silica-titania, chromium-silica, silica alumina, silica-titania-chromium, aluminum phosphate, alumina and mixtures thereof. 6. The method of claim 1 wherein said organic solvent is an alcohol. 7. The method of claim 6 wherein said alcohol is a C 5 -C 6 alcohol. 8. The method of claim 1 wherein said alcohol is l- hexanol. 9. The method of claim 4 wherein said calcined xerogel is subsequently activated by heating in air at a temperature of 400 to 900°C.

Inventors: KITAGUCHI JOEL LEE, MORAN DARVIN RANDAL, MULLEN JEFFREY DONALD, KITAGUCHI, JOEL, LEE, MORAN, DARVIN, RANDAL, MULLEN, JEFFREY, DONALD
Applicants: GRACE W R & CO, W.R. GRACE & CO.-CONN.
Application year: 1994
Main classification: B01J37
IPC codes: B01J23, B01J37, C08F4